1. Field of the Invention
The present invention relates to a sequence control apparatus for generating a plurality of operation commands for performing respective events of operations in a predetermined sequence on a controllable device, and more particularly to an arrangement for improvement in the flexibility or versatility of such a sequence control apparatus whose major portion is constituted by a computer.
2. Discussion of the Prior Art
A large number of actuators are used in a machining line which includes a machine tool, a work supply device, a work loading/unloading device and a work delivery device. For automatically controlling such a machining line as a controllable device having many controllable actuators to perform a series of operations, appropriate operation commands must be sequentially applied to the actuators in the predetermined order. For this purpose, a sequence control apparatus is usually employed. The sequence control apparatus is not adapted merely for generating the operation commands in the predetermined sequence, but is required to check the operating conditions of various components of the controllable device that are driven or operated by the actuators, in order to assure the desired sequential operations without interference of the operating components with each other. In other words, the sequence control apparatus is required to start each actuator only after the specific condition for permitting a normal operation of the actuator is satisfied. Further, even after the actuator is started, it is necessary to interrupt the operation of the actuator if there arises any abnormality that prevents the normal operation of the actuator. That is, a certain condition should be satisfied during the operation of the actuator. The former condition is referred to as "start condition" which should be satisfied to start an actuator, and the latter condition is referred to as "continuing condition" which should be satisfied for the actuator to continue its operation. These start and continuing conditions are collectively referred to as "interlock condition".
Recently, a computer is utilized as a major portion of a sequence control apparatus as described above. The computer functions to determine the order in which a plurality of actuators of a controllable device are activated, and monitor the operating conditions of the controllable device for checking whether the appropriate interlock conditions are satisfied for the respective actuators. The sequence control apparatus using a computer is easier and economical to manufacture, than a hard-wired sequence controller which utilizes many relays.
However, a computer-aided sequence control apparatus requires different control programs adapted for exclusive use for different types or configurations of the controllable device. The preparation of the exclusive control programs requires a considerable time. Further, the control program for a given controllable device should be modified if the events of operations to be performed or the interlock conditions for the operations are changed. While the modification of the control program is easier than the corresponding modification of the circuitry of a hard-wired sequence control apparatus, the modification of the control program requires considerable efforts of an expert programmer.
It is noted that the principle of the present invention is applicable not only to a device which includes an actuator or actuators for producing mechanical movements of movable components, but also to a device in which the components operate to perform a non-mechanical operation. In the present application, the term "operation" or "events of operation" should be broadly interpreted so as to encompass both the mechanical movements and the non-mechanical operation as indicated above.
In a computer-aided sequence control apparatus as described above, an operation of the controllable device is interrupted in the event the interlock condition for that operation is lost for some reason or other. In this case, the cause for the interruption should be located. However, it is difficult to locate the abnormality that causes the interruption of the operation. Namely, the operating conditions of the individual components of the controllable device should be checked against the relevant portion of the control program. This checking must be done by an expert who is versed in the control program, and requires large amounts of time and efforts.
For easy maintenance and inspection of the controllable device, it is desirable that the actuators of the controllable device be operable in a stepping mode by the operator using an operator's control unit. In view of this desire, the sequence control apparatus is generally provided with such an operator's control unit for permitting a manual stepping operation of the controllable device. In this case the operator's control unit has many operator-controlled switches, which are operated by the operator in a predetermined sequence. However, the operator is not usually thoroughly familiar with the predetermined sequence of operation, and must consult the operating manual for the correct stepping procedure. This also requires considerable time and efforts.